Yes, Harold has it right..Build an 'open-grid' bench the very same dimensions as the ply, using "1 x 4"s and 2"x 3" legs ..Then add risers to the cross members of the grid, tall enough above cross members using 1"x 3" lumber to hold entire ply high enough to have track and trains to go under the ply. Begin with the main line descending beginning at about where the word 'joint' is written on the right, by making a 'cookie cutter' cut in the ply. The trestle will then become lower than depot, on a grade. Continue track to under ply...Begin ascent up, under Drago, to tunnel portal, again using risers screwed to cross members...
Bear this in mind..If the layout is free standing you're OK. But, if it is against the wall/s you can have lots of problems reaching across to rescue a stalled/derailed train or work on scenery at East Ridge or Drago.. Your leaning over to do this will be tough. Your sleeves and arms can break things under them... Ex: Reaching in to East Ridge could spell disaster to the trestle under you..And reaching across the yard to Drago will easily cause havoc to cars in the yard.
Finally, I'd move the engine house to that spur above it and have prior EH track depict a thru-main-line that connects your RR to the outside world. Center track of the 3 then becomes an interchange track..(cars to in/out of 'your' road).
OR !... Scrap the entire design and play it safe with a shelf and/or around the room design... Allot can be modeled on a 1 foot deep shelve and makes everything very very easy to work with in any scale, gauge, or type of RR..Use a wye, turntable or balloon track at one or both ends to turn equipment around..
Hope this helps, M, Los Angeles